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RESEARCH ARTICLE
828 Aviation, Space, and Environmental Medicine x Vol. 84, No. 8 x August 2013
R EIS C, M ESTRE C, C ANHÃO H. Prevalence of fatigue in a group of
airline pilots. Aviat Space Environ Med 2013; 84:828 – 33.
Background: Fatigue is a common phenomenon in airline pilots that
can impair alertness and ability of crewmembers to safely operate an
aircraft and perform safety related tasks. Fatigue can increase the risk of
an incident or even an accident. This study provides the fi rst prevalence
values for clinically signifi cant fatigue in Portuguese airline pilots. The
hypothesis that medium/short-haul pilots may currently present different
levels of fatigue than long-haul pilots was also tested. Methods: A survey
was conducted by requesting Portuguese airline pilots to complete
questionnaires placed in the pilots ’ personal lockers from 1 April until
15 May 2012. The questionnaire included the self-response Fatigue Se-
verity Scale (FSS) to measure subjective fatigue and some additional
questions concerning perception of fatigue by pilots. Results: The preva-
lence values for total and mental fatigue achieved in the Portuguese air-
line pilots were: 89.3% (FSS ⱖ 4) and 94.1% (FSS ⱖ 4) when splitting the
sample in two subsamples, long- and medium/short-haul pilots. Levels
of total and mental fatigue were higher for medium/short-haul pilots.
Discussion: The analysis of fatigue levels in each type of aviator showed
that medium/short-haul pilots presented the highest levels of total and
mental fatigue. This study produced the fi rst prevalence values of total
and mental fatigue among Portuguese airline pilots, which represents a
great step to understanding and addressing this critical phenomenon.
Keywords: aviation , fatigue , prevalence , pilots .
F
ATIGUE IS AN important factor in modern aviation,
usually attributed to long duty periods, disruptive
schedules, circadian disruptions, and sleep deprivation
( 3 ). There are several defi nitions of fatigue in the litera-
ture; however, this paper discusses fatigue in the context
of the commercial aviation industry. Fatigue is defi ned
by the International Civil Aviation Organization, the en-
tity responsible for supervising civil aviation, as “ a physi-
ological state of reduced mental or physical performance
capability resulting from sleep loss or extended wake-
fulness, circadian phase, or workload (mental and/or
physical activity) that can impair a crewmember ’ s alert-
ness and ability to safely operate an aircraft or perform
safety related duties ” ( 12 ).
Over the last 30 yr, air traffi c around the world has
doubled every 10 yr. Total costs have also risen. Begin-
ning in the early eighties, the cost of fuel was the main
factor. But after the terrorist attacks of September 2001,
the costs associated with passenger security created
an additional expense ( 8 , 17 ). The “ low cost/low fares ”
model, which appeared in the late seventies, has also in-
creased airline competition to reduce fares. The combi-
nation of rising costs and reduced fares in the last 10 yr
has created an emphasis on effi ciency and productivity.
Reducing the costs of labor is a number one priority for
most companies and also translates to extended work/
fl y periods for aircrew. The maximum duty and mini-
mum rest present in fl ight and rest legislation are “ Goals
of Productivity ” for the majority of airlines around the
world ( 8 , 17 ). As such, it is not surprising that fatigue is a
clear and real risk among commercial airline pilots.
NASA ’ s Aviation Safety Reporting System indicates that
21% of reported aviation events are fatigue related ( 24 ).
While general fatigue is important for any professional,
mental fatigue is extremely important in this profes-
sional group, taking into account all the demands of safe
airline fl ight. Mental fatigue refers to a feeling that can
be experienced during or after long periods of cognitive
activity. It involves fatigue, exhaustion, decreased level
of commitment to the task, and may even lead to a strong
will to stop performing ( 1 , 4 ). Pilot ’ s fatigue is a genuine
concern in terms of safety, health, effi ciency, and pro-
ductivity ( 9 ), and the quantifi cation of this phenomenon
is of major importance to the pilots ’ well-being and pas-
senger ’ s safety.
Aviator fatigue is common and well discussed by the
scientifi c community ( 5 , 20 , 25 ), noting that it decreases
performance and increases the risk for an incident or
even an accident ( 6 ). In a recent study of fatigue in United
Kingdom (UK) pilots, 71% of those who fl ew medium-
haul fl ights reported fatigue. More concerning were the
data that 81% of those reporting fatigue claimed that
their fatigue was worse than 2 yr prior ( 13 ). The sample
may not have been wholly representative of the pilots ’
population from the Professional Pilot ’ s Rumours Net-
work ( www.pprune.com ) website, since it only covered
162 individuals from a population of 49,500 registered
pilots. Nevertheless, a high level of fatigue was reported
and an increase in the burden of fatigue was also sug-
gested ( 13 ). Similarly, in a study developed in a British
airline company, Houston et al. ( 10 ) refer in their work to
a fatigue reporting incidence value of 103 per 1000 pilots
per year.
There are two different types of fl ights: medium/
short-haul and long-haul fl ights. Medium/short-haul
operations are fl ights with less than 6 h duration with
several sectors in one duty period. Long haul operations
are fl ights with 6 or more hours, usually with one or two
From the Rheumatology Research Unit, Instituto de Medicina Molecu-
lar, Faculdade de Medicina, Universidade de Lisboa, Lisbon, Portugal.
This manuscript was received for review in September 2012. It
was accepted for publication in February 2013.
Address correspondence and reprint requests to: Cátia Reis, 11°A
1600-896 Lisboa, Av. Egas Moniz, Lisboa, Portugal; reis.catia@gmail.
com .
Reprint & Copyright © by the Aerospace Medical Association,
Alexandria, VA.
DOI: 10.3357/ASEM.3548.2013
Prevalence of Fatigue in a Group of Airline Pilots
Cátia Reis , Catarina Mestre , and Helena Canhão
Aviation, Space, and Environmental Medicine x Vol. 84, No. 8 x August 2013 829
FATIGUE IN AIRLINE PILOTS — REIS ET AL.
sectors maximum. Due to their distinct characteristics,
the two types of fl ight operations may create different
levels of fatigue. Therefore, we hypothesized that me-
dium/short-haul pilots may currently present different
levels of fatigue from long-haul pilots, based on the fact
that these pilots have less total time in low phases of
fl ight than long-haul pilots ( 15 , 22 ). To test that hypoth-
esis, we used the 9-item Fatigue Severity Scale (FSS) ( 14 ),
one of the most commonly used self-report questionnaires
to measure fatigue. This work aimed to: 1) identify and
quantify the prevalence value of self-reported total and
mental fatigue in the community of Portuguese pilots;
and 2) assess and compare the level of fatigue in the two
most common types of fl ight: medium/short-haul and
long-haul.
METHODS
Study Population
From a questionnaire applied to a total population
of approximately 1500 commercial airline pilots working
in Portuguese airlines, 456 valid responses were obtained.
The inclusion criteria were: being an airline pilot on ac-
tive duty (Commanders and First Offi cers), ages between
20 and 65 yr old, and having fl own during the last 6 mo.
Questionnaire (Validation)
A pretest was used to validate the survey, which was
adapted to the Portuguese airline pilot population. This
pretest was answered by a sample of 104 pilots from
mid-January to mid-February 2012. The nine items
(questions) investigate the severity of fatigue in differ-
ent situations during the previous 2 wk. Scores range
from 1 to 7, where 1 indicates strong disagreement and
7 strong agreement. The fi nal score represents the mean
value of these nine items. The cutoff value established as
clinically signifi cant for fatigue in this scale is 4 ( 7 , 11 , 14 ).
Although the FSS had already been validated for the
Portuguese general population ( 16 , 18 ), it was decided to
validate it for the Portuguese airline pilots ’ population.
The results of the validity and reliability analysis sug-
gested the FSS is a reliable measure to evaluate fatigue
in the study population (Mestre C; unpublished obser-
vations; 2012 ).
Final Questionnaire
The fi nal questionnaire also requested socio-demo-
graphic data, scales of psychological evaluation, and
questions concerning labor conditions. In order to un-
derstand the perception that pilots have in relation to
fatigue, four questions were asked: “ Do you feel so tired
that you think you should not be on controls? ” ; “ Have you
ever reported yourself unfi t for fl ight as a result of ac-
cumulated fatigue? ” ; “ Has it ever happened that you have
made mistakes in the cockpit as a direct consequence of
fatigue? ” ; and lastly “ How many Human Factors Confi -
dential Reports have you made on the last 6 months? ”
A random number was assigned to each inquiry to en-
sure that all of them were distributed by the investigating
team, thus preventing duplication and consequent fraud.
All questionnaires were put in the personal locker of
each pilot, a sample representing all Portuguese airlines.
Upon being answered, the inquiries were deposited in a
locked ballot box. The pilots were told that the inquiries
should be answered from 1 April until 15 May. Of the
1498 inquiries distributed, 456 were correctly answered,
23 were invalidated due to duplication or for being in-
correctly fi lled, and 1019 were not answered ( Fig. 1 ) .
Statistical Analysis
The minimum sample size required in the analysis to
calculate the prevalence of the study population was
based on the prevalence value achieved through the
pretest (86.5%). The equation for sample size calculation
for fi nite populations was used, based in the estimated
proportion of occurrence, for a confi dence interval of
95% and a critical value of 1.96 ( 26 , 29 ). A fi nal value of
approximately 180 individuals minimum was obtained.
The sample was analyzed regarding medium/short-
haul fl ights, long-haul fl ights, and total sample (which
included both types of fl ight). An exploratory factor
analysis was applied to the FSS scale. Values of Cron-
bach a ⱖ 0.6 and factorial loads ⱖ 0.5 were considered
acceptable, bearing in mind a preliminary exploratory
study. The comparison of fatigue levels between the two
groups was performed using the Mann-Whitney test.
A signifi cance level of 5% was considered in all the sta-
tistical analysis undertaken. Statistical analyses were
carried out with Statistica v.10 (Stat Soft. Inc., Tulsa, OK;
2011).
Ethical Considerations
The Portuguese Pilot ’ s Association agreed to the
development of the study. In addition, the National
Commission of Data Protection was contacted and gave
their approval. The Pilots Association announced the
study to all pilots of Portuguese airlines and explained
the importance and the aim of the study. Also, the ques-
tionnaire had brief instructions explaining how to an-
swer and clarifying the objectives of the study. Informed
consent was not required because all pilots were invited
to answer the questionnaire voluntarily and anony-
mously. Furthermore, no interaction occurred between
the pilots and the researchers to jeopardize individual
privacy.
RESULTS
Table I shows the baseline characteristics of the vali-
dation test and the study population, where the similar-
ity between the two samples may be observed. For the
study population the mean age was 39.31 6 8.39 yr, 14
(3.1%) of which were women and 442 (96.9%) were men.
In the validation sample the mean age was 40.9 6 9.02 yr,
with 3 (2.9%) being women and 101 (97.1%) being men.
These numbers correspond roughly to the female/male
ratio of Portuguese airline pilots. Regarding the type
of fl ight, we obtained 127 (27.9%) valid responses for
the long haul and 314 (68.9%) for medium/short haul, a
830 Aviation, Space, and Environmental Medicine x Vol. 84, No. 8 x August 2013
FATIGUE IN AIRLINE PILOTS — REIS ET AL.
proportion similar to the total number of airline pilots in
Portugal. In the validation study identical proportions
were also obtained, 80 (77.7%) for the medium/short
haul and 24 (22.3%) for the long haul. A small number of
pilots, 15 (3.3%), do both types of fl ight, although no re-
spondents in this group were part of the validation sam-
ple. With respect to the professional category, there were
59 (57.3%) Commanders and 44 (42.7%) First Offi cers
on the validation sample. However, better results were
obtained for the study sample, with a proportion of
234 (51.3%) Commanders to 222 (48.7%) First Offi cers.
Concerning working times, values obtained were very
similar for both samples, with values of duty hours in
the validation sample of 98.50 6 38.43 h and in the study
sample of 98.47 6 32.52 h; the number of fl ight hours in
the validation sample were 59.50 6 18.19 h, and in the
study population was 60.24 6 18.46 h; the number of
sectors fl own for the validation sample was 23 6 16.61
and for the study sample was 22.07 6 12.9. All values
relating to working times were calculated for 28 d.
Fig. 1. Selection of the study population.
TABLE I. BASELINE CHARACTERISTICS OF THE VALIDATION AND STUDY AIRLINE PILOT POPULATIONS.
Validation Study
Data Collection 15/01/2012 to 15/02/2012 1/04/2012 to 15/05/2012
Sample
Dimension 104 456
Age (mean 6 SD) 40.9 6 9.02 39.31 6 8.39
Sex
Male (%) 101 (97,1%) 442 (96,9%)
Female (%) 3 (2,9%) 14 (3,1%)
Professional Category
Commanders (%) 59 (57,3%) 234 (51,3%)
First Offi cers (%) 44 (42,7%) 222 (48,7%)
Type of Flight
Medium/Short Haul (%) 80 (77,7%) 314 (68,9%)
Long Haul (%) 23 (22,3%) 127 (27,9%)
Medium/Short 1 Long Haul (%)
0 (0%) 15 (3,3%)
Working Times
Duty Hours (28 d) (mean 6 SD) 98.50 6 38.43 98.47 6 32.52
Flight Hours (28 d) (mean 6 SD) 59.50 6 18.19 60.24 6 18.46
Sectors fl own (28 d) (mean 6 SD) 23.00 6 16.61 22.07 6 12.90
Aviation, Space, and Environmental Medicine x Vol. 84, No. 8 x August 2013 831
FATIGUE IN AIRLINE PILOTS — REIS ET AL.
Regarding validation testing, the exploratory factor
analysis applied to the FSS for the Portuguese pilots ’
population presented two dimensions: mental and physi-
cal fatigue. The distinction between the two dimensions
was made by considering the content of the items as-
signed to each factor as shown in Table II . The variables
or items of the fi rst factor refer to mental or psychologi-
cal symptoms of fatigue, while the second factor variables
refer to physical symptoms of fatigue. Item 5 showed a
factorial loading greater than 0.5 in both factors regard-
ing its content of “ Fatigue causes frequent problems for
me, ” which fi t in both factors. Thus, for purposes of
score range, this item should be considered part of both
mental and physical fatigue. Regarding mental fatigue,
a factor with an eigenvalue of 4.489 was extracted, cor-
responding to 49.9% of the variance explained. Physical
fatigue showing a factor with an eigenvalue of 1.251 was
found, explaining 13.9% of the variance. The items were
distributed through the dimensions by the varimax ro-
tation method.
The reliability of the scale was assessed by measuring
internal consistency with Cronbach a , where acceptable
values of Cronbach a ⱖ 0.6 were identifi ed in this pre-
liminary exploratory study. Our results showed that the
chosen scale was a reliable measure of these phenomena
in the present population, presenting a Cronbach a 5
0.856 for the total scale. Values were obtained for the
two subscales: for mental fatigue a Cronbach a 5 0.887
and for physical fatigue a Cronbach a 5 0.598, where
the subscale of physical fatigue was a nonsatisfactory
value of Cronbach a . Regarding the score for the two
subscales, it was calculated through the average of the
sum of the items considering a cutoff value of 4, as sug-
gested in the literature for the total scale ( 16 , 28 ). For the
validation study, the prevalence of clinically signifi cant
total fatigue was 86.5% and for mental fatigue was 87.5%
(FSS ⱖ 4) for these pilots (Mestre C; unpublished obser-
vations; 2012 ).
Concerning the study population, the number of pi-
lots who fl ew both types of fl ights was small ( N 5 15),
and for that reason those values were only included when
analyzing the total sample, but were not considered as
a separate group. For the total sample of Portuguese
airline pilots, a percentage value of total fatigue of 89.3%
(FSS ⱖ 4) was obtained. For those pilots who fl ew me-
dium/short haul, a percentage value for total fatigue of
93% was determined, and a percentage value for total
fatigue of 84.3% was obtained for those who fl ew long
haul. The comparison of the indices of total fatigue of
the two groups (medium/short haul and long haul)
showed statistically signifi cant differences between the
group of pilots who fl ew medium/short haul and the
group of pilots who fl ew long-haul fl ights (Mann-Whitney
U 5 15,772.50; P 5 0.001). Pilots who fl ew medium/short-
haul fl ights presented higher values of clinically signifi -
cant fatigue ( Fig. 2 ). Distribution analysis of the samples
( Fig. 2 ) showed that the obtained values were not in the
proximity of the cutoff point (FSS ⱖ 4) but above it.
Regarding mental fatigue, a percentage value of 94.1%
(FSS ⱖ 4) was obtained for the total sample of Portu-
guese airline pilots, 96.5% for medium/short-haul pilots,
and 92.1% for long-haul pilots. The comparison of the
indices of mental fatigue of the two groups once again
yielded statistically signifi cant differences between long-
haul and medium/short-haul pilots (Mann-Whitney U 5
15,882.50; P 5 0.001), with the latter presenting the high-
est values of clinically signifi cant mental fatigue ( Fig. 3 ).
The distance between the majority of the distribution of
the samples ( Fig. 3 ) and the cutoff point (FSS ⱖ 4) was
even more evident for the medium/short-haul pilots.
Concerning fatigue perception, when asking pilots
“ Do you feel so tired that you think you should not
be on controls? ” , 60 (13.2%) said that never happened,
87 (19.1%) replied that happened only once, 234 (51.3%)
said a few times, and 75 (16.4%) said that happened fre-
quently. Considering the question “ Has it ever happened
that you have made mistakes in the cockpit as a direct
consequence of fatigue? ” , 417 (91.4%) pilots claimed this
had occurred. Regarding the pilots reporting fatigue
( “ Have you ever reported yourself unfi t for fl ight as a
result of accumulated fatigue? ” ), 372 (81.6%) never did,
50 (11%) reported only once, 28 (6.1%) reported a few
TABLE II. FACTOR STRUCTURE OF THE FATIGUE SEVERITY SCALE
USING PRINCIPAL COMPONENT ANALYSIS.
Statements Factor 1 Factor 2
1. My motivation is lower
when I am fatigued.
0.64
2. Exercise brings on my fatigue 0.82
3. I am easily fatigued. 0.73
4. Fatigue interferes with my
physical functioning.
0.79
5. Fatigue causes frequent problems for me. 0.58 0.52
6. My fatigue prevents sustained
physical functioning.
0.77
7. Fatigue interferes with carrying out
certain duties and responsibilities.
0.86
8. Fatigue is among my three
most disabling symptoms.
0.86
9. Fatigue interferes with my work,
family, or social life.
0.88
Fig. 2. Comparison of total fatigue for medium/short haul and long
haul pilots (* P , 0.05).
832 Aviation, Space, and Environmental Medicine x Vol. 84, No. 8 x August 2013
FATIGUE IN AIRLINE PILOTS — REIS ET AL.
times, and 6 (1.3%) reported frequently. Finally, to the
question “ How many Human Factors Confi dential Re-
ports have you made in the last 6 months? ” , the great
majority had not generated any reports: 371 (81.4%) had
not and the mean value of reports made was 0.38 6 1.15
( Table III ) .
DISCUSSION
Until now, prevalence values of fatigue had not been
calculated for airline pilots in Portugal and, because of
that, it was decided to perform a validation pre-test.
During the validation for the Fatigue Severity Scale, a
total fatigue score was confi rmed as reliable. Further
analysis established two fatigue dimensions; one for
mental fatigue and the other for physical fatigue. In the
case of mental fatigue, a very robust value of Cronbach
a 5 0.887 was determined. We suspect this population is
highly affected by mental fatigue, as described in previ-
ous papers ( 9 , 24 ), where increased reaction times, short
term memory loss, impaired judgment, poorer decision-
making, and decreased visual perception have been
noted ( 9 , 23 , 24 ). All the frequency values reported for fa-
tigue were higher than expected [total fatigue 89.3%
with scores of 5.03 6 0.94 (mean 6 SD) and mental fa-
tigue 94.1% with scores of 5.62 6 0.91 (mean 6 SD)]. In
a validation study of several Swiss populations, 18% of
healthy subjects reported clinically signifi cant fatigue
with scores of 3.00 6 1.08 (mean 6 SD). Swiss patient
cohorts in this study showed higher fatigue scores: 69%
of those with multiple sclerosis scored 4.66 6 1.64, 49%
of those with ischemic stroke scored 3.90 6 1.85, and
62% with sleep-wake disorders scored 4.34 6 1.64 (all
means 6 SD) ( 28 ). All the results obtained for the pilots ’
population showed a fatigue frequency well above the
results obtained in these populations, which confi rmed
the complaints made by this professional class regard-
ing fatigue. One of the major problems when calculating
frequency values is the response rate of subjects, but in
this study, it was possible to achieve a large sample size
( N 5 456), which was 30.6% of the 1498 members of the
pilot group studied and higher than in other papers ( 13 ).
Another study involving airline pilots and using the
same evaluation scale was completed by the British Air-
line Pilots Association. In this study 45% of pilots re-
ported signifi cant fatigue ( 27 ). The British Airline Pilots
Association 2011 study used a cutoff point of 5, different
from the one used for the Portuguese pilot population
(FSS ⱖ 4). But, even if the cutoff point for the Portuguese
population was 5, the frequency for signifi cant fatigue
would still surpass that value (59.6%).
A statistically signifi cant level of total and mental fa-
tigue was obtained between different types of fl ights,
with medium/short haul presenting the highest levels
Fig. 3. Comparison of mental fatigue for medium/short haul and long haul pilots.
TABLE III. PERCEPTION AND REPORT OF FATIGUE AMONG PORTUGUESE AIRLINE PILOTS.
Question Answer
“ Do you feel so tired that you think you
should not be on controls? ”
Never 60 (13.2%) Once 87 (19.1%) Few times 234 (51.3%) Frequently 75 (16.4%)
“ Has it ever happened that you have made
mistakes in the cockpit as a direct
consequence of fatigue? ”
Yes 417 (91.4%) No 39 (8.6%)
“ Have you ever reported yourself unfi t
for fl ight as a result of accumulated fatigue?
Never 372 (81.6%) Once 50 (11%) Few times 28 (6.1%) Frequently 6 (1.3%)
“ How many Human Factors Confi dential
Reports have you made in the last six months? ”
Min. Max.
Mean 6 SD
014
0.38 6 1.15
371 (81.4%) 1 (0.2%)
Aviation, Space, and Environmental Medicine x Vol. 84, No. 8 x August 2013 833
FATIGUE IN AIRLINE PILOTS — REIS ET AL.
of fatigue compared to long haul. These data are consis-
tent with several studies over the last years, where pilots
who fl ew medium/short-haul fl ights reported higher
levels of fatigue ( 2 , 19 , 22 ), which could be a consequence
of diminished resting periods ( 15 , 22 ). Long-haul pilots
are known to have fatigue due to overnight fl ights, jet lag,
and interference with the sleep/wake cycle and body
temperature rhythm ( 5 , 21 , 23 ).
Fatigue is a subjective concept which may vary in each
individual. The evaluation instrument (FSS) is a self-
response questionnaire and our study did not attempt to
control the timing for each individual response (before,
during, or after a fl ight). This presents inherent subjec-
tivity and limitations. Nonetheless, it is a widely used
survey and it has been proven effective in the measure-
ment of subjective fatigue. When trying to understand
the perception of fatigue in Portuguese pilots, the major-
ity of pilots who responded showed good awareness
of his/her level of fatigue. However, when it came to
documenting their fatigue to the airline company (using
a Human Factors Confi dential Report), they preferred
not to report.
In this study, the prevalence of total and mental fa-
tigue among Portuguese airline pilots was established
in a solid sample of responders to our survey. Our data
support the belief that fatigue among pilots is one of the
more pressing safety issues of today ’ s aviation industry.
Fatigue management is essential to maintain the safety
of crewmembers and the traveling public during fl ight
operations. By quantifying the level of fatigue in Portu-
guese airline pilots, we have establish crucial baseline
data and hope to create continued interest among Por-
tuguese pilots, airline companies, and aviation regula-
tors to fi nd ways to reduce the risk to fl ight safety in the
future.
ACKNOWLEDGMENTS
The authors wish to acknowledge the great importance of the
involvement of the Portuguese Airline Pilots Associations, Associação
Portuguesa de Pilotos de Linha Aérea (APPLA) and Sindicato de
Pilotos da Aviação Civil (SPAC), for their full support, especially by
highlighting the importance of this study among the pilot community.
Authors and affi liation: Cátia Reis, M.Sc., Catarina Mestre, M.Sc., and
Helena Canhão, M.D., Ph.D., Rheumatology Research Unit, Instituto
de Medicina Molecular, Faculdade de Medicina, Universidade de
Lisboa, Lisbon, Portugal.
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